Semiconductor lasers have always required special techniques for controlling their oscillation wavelengths, and in many cases, techniques for achieving a single-wavelength of oscillation. Nevertheless, such results have been difficult to obtain reliably, particularly when the laser is pulsed. One of the techniques which has been employed is to use a periodic variation of index of refraction, to produce the effect of a continuous diffraction grating, or nearly continuous grating, substantially coplanar with the lasing junction. The result was frequently oscillations which occurred at either or both of two wavelengths on either side of a "stop band", which was centered around the desired center wavelength, commonly called the Bragg wavelength.
Oscillation at the Bragg wavelength has been achieved by introducing a small, typically "quarter-wave length", discontinuity in the center of the otherwise continuous grating. It has been found that making commercial semiconductor lasers this way results in a very low yield, i.e., only a few are selected for use and the remainder are discarded.
Another technique achieves oscillation stably at one of the wavelengths at the edge of the stop band by introducing another sort of axial nonuniformity in the distributed feeback laser, such as shaping the refractive index or gain profile along the path of the oscillations by making one of the end faces of the semiconductor significantly less reflective than the other. It has been found that even this technique may suffer from the random locations of the end faces of the laser with respect to the phase of the period of the grating. For a description of the general problem associated with random location of the end faces, see T. Matsuoka et al, "Verification of the Light Phase Effect at the Facet on DFB Laser Properties", IEEE Journal of Quantum Electronics, QE-21, pp. 1880-1886 (1985).
Nevertheless, some significant stabilization and wavelength tuning results are disclosed in a pair of recent articles "Broad Wavelength Tuning Under Single-Mode Oscillation With A Multi-Electrode Distributed Feedback Laser" by Y. Yoshikuni et al, in Electronics Letters, Vol. 22, No. 22, Oct. 23, 1986, pp. 1153-1154 and "Improvement of Single Longitudinal Mode Stability by Gain Profile Control in DFB LD" by M. Yamaguchi et al, in 10th IEEE International Semiconductor Laser Conference, Kanazawa, Japan, October 1986, pp. 64-65. It thus appears that the dual electrode structure of the devices disclosed in those references provides an alternate way of shaping the refractive index or gain profile of distributed feedback lasers.
Nevertheless, it is apparent from a close reading of these articles that both lasers were carefully selected initially to oscillate at a single-wavelength before the split electrode structure was provided. Thus, the same problems of low yields will probably result from these techniques as were experienced heretofore. Moreover, a full amplitude modulation of those lasers to produce the pulses typically desired in an optical fiber-based communication system will tend to be slow because of the large changes in carrier concentrations required, and in the general case will be accompanied by significant undesired "chirp" of the oscillation wavelength.
Accordingly, it is an object of this invention to achieve rapid pulsing of distributed feedback lasers at discrete single-wavelengths and to minimize the frequency change or "chirp" of such wavelengths. Another object of this invention is to employ laser devices of the distributed feedback type which are initially likely to oscillate at either, or both, wavelengths at the sides of the stop band and, therefore, would previously have been discarded.
A further object of this invention is to provide a transmitter for an optical communication system in which a distributed feedback laser is modulated at a higher information bit rate and with less chirp than heretofore possible.